XT Di2 RD (GS) capacity

[Bike Whisperer]

I have a feeling that the max capacity is allowing for the normal 11-36 on old school 2x drivetrains. That would be in the GS's capacity.

 

I think it's just poorly put together.

Incidentally, I am using the PG-1170 cassette, 11spd, 11-36 currently as this gives me enough gear range on my 2x11. But it's SRAM, and Shimano would design for their own cassettes.

 

This does create a 35T capacity.

 

 

 

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[JXV]

I have a confusing question... I'm looking at buying the new XT Di2, but...

 

The new XT Di2 RD is only available as a GS (Medium Cage) and claims to have a max capacity of 35T, which seems normal for a GS RD. However, Shimano markets the XT Di2 as being both 1x11 and 2x11 capable. They say you can use the 11-46 cassette for the 1x11 and either the 11-42 or 11-40 for a 2x11 setup.

 

From Shimano product page for RD-M8050-GS:

• Series : SHIMANO DEORE XT
• Maximum Sprocket : "42T(2x11-speed) 46T(1x11-speed)"
• Minimum Sprocket : 11T
• Front Difference : 10T
• Total Capacity : 35T

Here's my dilemma. Even if one uses the new shimano 10 tooth difference crank (say 36/26) both the claimed 2x11 setups exceed the maximum capacity of the RD.

 

Using the 11-40 cassette and the 36/26 chainrings (which needs less capacity than with the 42T cassette) this gives us a 10+29 needed total capacity of 39T. The school I went to thought me that 39>35. With a 42T cassette, the needed capacity climbs to 41T. So theoretically the XT Di2 RD won't handle the stated 2x11 setups based on the stated capacity.

 

What am I missing? Do I need to get the SGS XTR Di2 RD for this to work as 2x11? Does this only apply to some programmed SynchroShift which manages the total capacity?

 

So before you ask why not just do 1x11, my other bike is a 1x11, ... I like my short gear spacings for long rides (ex roadie)

 

Any ideas? Anyone from Shimano care to shed some light on this?

Download the Shimano Specifications pdf from si.shimano.com then open it and go to the DI2 chapter and select the RD specs page.

 

The XT GS RD does indeed have a 35t total capacity. There is a note attached that explains how the Di2 shift controller automatically prevents selection of the cross-chained ratios that would result in hypo/hyper extension of the RD.

 

In a 2x11 config it will not allow you to select the large rear cog with the big ring ..... it will first shift down to the smaller front ring before moving to the larger rear cogs.

 

Same for the faster gears. It won't permit the 11cog to be selected with the small front ring.

 

So the controller effectively limits overlaps. You still get the use of the full span of front and rear cog ratios but those that cause cross-chaining are avoided (coincidentally these are also the ones that cause too little or too much RD extension). This is the reason why the Di2 GS RD can give you the full range of ratios with a lower capacity.

 

In the 2x11 config you can have up to 11-42 cassette and in 1x11 you can have up to 11-46 with the Di2 XT RD.

 

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[Bike Whisperer]

Download the Shimano Specifications pdf from si.shimano.com then open it and go to the DI2 chapter and select the RD specs page.

 

The XT GS RD does indeed have a 35t total capacity. There is a note attached that explains how the Di2 shift controller limits cross-chaining.

 

In a 2x11 config it will not allow you to select the large rear cog with the big ring ..... it will first shift down to the smaller front ring before moving to the larger rear cogs.

 

Same for the faster gears. It won't permit the 11cog to be selected with the small front ring.

 

So the controller effectively limits overlaps. You still get the use of the full span of front and rear cog ratios but those that cause cross-chaining are avoided (coincidentally these are also the ones that cause too little or too much RD extension). This is the reason why the Di2 GS RD can give you the full range of ratios with a lower capacity.

 

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Thanks, that would make sense. I assumed that there is some SynchroShift management of this to prevent you from ripping off the RD by cross-gearing.

 

I'll have a look at the details you sent.

[Captain Fastbastard Mayhem]

Download the Shimano Specifications pdf from si.shimano.com then open it and go to the DI2 chapter and select the RD specs page.

 

The XT GS RD does indeed have a 35t total capacity. There is a note attached that explains how the Di2 shift controller automatically prevents selection of the cross-chained ratios that would result in hypo/hyper extension of the RD.

 

In a 2x11 config it will not allow you to select the large rear cog with the big ring ..... it will first shift down to the smaller front ring before moving to the larger rear cogs.

 

Same for the faster gears. It won't permit the 11cog to be selected with the small front ring.

 

So the controller effectively limits overlaps. You still get the use of the full span of front and rear cog ratios but those that cause cross-chaining are avoided (coincidentally these are also the ones that cause too little or too much RD extension). This is the reason why the Di2 GS RD can give you the full range of ratios with a lower capacity.

 

In the 2x11 config you can have up to 11-42 cassette and in 1x11 you can have up to 11-46 with the Di2 XT RD.

 

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Awesome.

 

The only thing that prevented me from asserting this was the potential for user programming. Glad that that isn't an issue, cos, well, ppl are stupid.

 

Tx for the clarification, J

[Bike Whisperer]

Update...

 

Based on JXV's input and suggestion I had a closer look at something I looked at before, but frankly ignored... When using the GS RD (only available option for XT Di2), there are restrictions programmed in the system to cater for the lack of capacity. The system will decide whether to allow a change or not, based on the front and rear gear combination. So if the combination falls outside of the RD's slack take-up capacity, it will not shift (when in manual mode). In SynchroShift mode it obviously caters for this in a smooth sequential manner, giving you the full range of gears available.

 

It will not shift into the small chainring if you are in any of the small 3 gears at the back as this would make the chain too slack. Also, will not shift into any of the 3 small gears in the back if you are in the little ring in front, for the same reason.

 

Actually, pretty clever stuff.

 

Thanks for all of your inputs into this topic... I'm no longer confused.

[JXV]

Awesome.

 

The only thing that prevented me from asserting this was the potential for user programming. Glad that that isn't an issue, cos, well, ppl are stupid.

 

Tx for the clarification, J

Maybe the XT version limits the amount of rider customisation permitted. In the XTR version of Di2 the SGS RD has 41T total capacity and could tolerate a greater degree of cross-chaining or a bigger difference in front ring sizes.

 

The XTR SGS RD permits use of a 3x11 system with the 11-40 cassette which yields a monstrous 660% gear range . The XT system only allows 2x11 and 1x11.

 

I presume the RDs would be coded so that the controller knows which type is fitted - since this would influence the permissible gear combinations.

 

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[Bike Whisperer]

yeah, but repeat after me....

 

People Are Stupid. 

I guess they did actually cater for the 'Stupid' factor with this, impressive.

[Bike Whisperer]

Maybe the XT version limits the amount of rider customisation permitted. In the XTR version of Di2 the SGS RD has 41T total capacity and could tolerate a greater degree of cross-chaining or a bigger difference in front ring sizes.

 

The XTR SGS RD permits use of a 3x11 system with the 11-40 cassette as well which permits a monstrous 660% gear range . The XT system only allows 2x11 and 1x11.

 

I presume the RDs would be coded so that the controller knows which type is fitted - since this would influence the permissible gear combinations.

 

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Exactly, this is what confused me as the SGS setup is adequate to work without programmed restrictions in shifting, but creates a lot of redundancy. I think this limited XT thinking is pretty ingenious giving you all the range without the duplication.

[s14phoenix]

I would run the sequential shifting setup. makes sense to run only 1 shifter then also.

 

... but then again I would be running 1x11 with a sram cassette for 10-42 and a relevant front ring for the terrain.

 

or just go 36T and HTFU....

[JXV]

Exactly, this is what confused me as the SGS setup is adequate to work without programmed restrictions in shifting, but creates a lot of redundancy. I think this limited XT thinking is pretty ingenious giving you all the range without the duplication.

If I had Di2 I can't imagine why I would ever choose manual shifting over the built-in Syncro-shift mode.....

Maybe only as a get me home option if one of the derailleurs broke

 

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[Bike Whisperer]

I would run the sequential shifting setup. makes sense to run only 1 shifter then also.

 

... but then again I would be running 1x11 with a sram cassette for 10-42 and a relevant front ring for the terrain.

 

or just go 36T and HTFU....

As stated before, I already have a 1x11 setup, with the e*Thirteen 9-44 cassette with 489% range. Pretty fine setup, but for multistage or ultras, I prefer closer / finer gear jumps, especially at the end. 20% gear jumps are just too brutal and I end up hunting for gears.

 

But the SynchroShift makes a lot of sense

[Bike Whisperer]

I'll let you know how it turns out once I get the Di2 bits...

 

Anyone want to buy 2x11 standard XT Shifters, FD and RD? 

[Captain Fastbastard Mayhem]

If I had Di2 I can't imagine why I would ever choose manual shifting over the built-in Syncro-shift mode.....

Maybe only as a get me home option if one of the derailleurs broke

 

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Same